Doppler, M. (2020). Search for scalar top quark pair production in the dilepton final state in 13 TeV pp collisions with the CMS experiment [Diploma Thesis, Technische Universität Wien]. reposiTUm. https://doi.org/10.34726/hss.2020.77221
The Standard Model of particle physics is one of the most successful and well-tested scientific theories. It encapsulates humankinds best understanding of how fundamental particles and forces are related to each other. However, the Standard Model of particle physics is found to be incomplete, as it does not provide explanations for cosmological observations such as dark matter and dark energy. Supersymmetry is a promising candidate for a theory beyond the Standard Model. For example, in many supersymmetric extensions of the Standard Model the lightest supersymmetric particle is weakly interacting and stable which makes it an excellent candidate for dark matter. So far, however, physics experiments have not been able to detect any supersymmetric signatures. A search for events with two charged leptons (electrons or muons) in the final state, arising from supersymmetric processes, is presented in this thesis. The analysis is based on proton-proton collision data taken by the CMS experiment at the CERN LHC in the operational period from 2016 to 2018 with a center of mass energy of 13 TeV. This amounts to an integrated luminosity of 137 fb1. In particular, scalar top quark pair production is considered. The tested models involve two jets originating from bottom quarks as well as the lightest neutralino in the final state, supplying a significant amount of missing transverse energy. The hypothetical supersymmetry events are efficiently separated from the dominant t t background with requirements on the significance of pmiss T and transverse mass variables. No significant deviation from the expected background is observed. The results are used to set limits on the production cross section of pair-produced top squarks. For the case where top squarks decay exclusively into a top quark and a neutralino, the exclusion limits on the mass of the lightest top squark are placed at up to 900 GeV and up to 440 GeV on the lightest neutralino at a 95% confidence level. For the decay involving an intermediate chargino, the mass limits at a 95% confidence level reach up to 850 GeV for the lightest top squark and up to 400 GeV for neutralino masses. For the top squark pair undergoing a cascade decay via charginos and sleptons, the exclusion limits reach up to 1.4 TeV for top squark masses and up to 900 GeV for the masses of the lightest neutralino at 95% confidence level. All in all, mass limits from previous analyses are improved by roughly 100 GeV.
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